From previous work we have concluded that the walls of the cardiac semilunar valves actively participate in opening and closing the valve leaflets. The structure of the walls and leaflets are consonant with this view. We propose to examine the structure or aortic valves with the electronmicroscope in a continuation of efforts to discover how natural valve leaflets, in contrast to all artificial devices that can be subsituted, escape fatigue and folding failures due to repetitive stresses. We also expect to produce models of diseased valves in animals and study how tissues are altered in the models as experimentally changed anatomy induces pathological responses. A structural comparison of venous arterial semilunar valves, including changes induced in venous tissues placed in an arterial circuit, is additionally proposed. Structural and pathological changes in tissues that follow altered stresses suggest that tissues are replaced periodically under normal conditions and can be caused to undergo abnormal replacement under pathological stresses. We expect to continue a radioautographic study of tissue replacement in valves, additionally using the electronmicroscope to identify the cells and tissues undergoing replacement. Tissue replacement should be greatest where stresses are most intense. Discovering those sites should help in understanding normal valve function as well as caring for diseased valves medically.